Preoperative Gabapentine for Carpal Tunnel: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Preoperative Gabapentine for Carpal Tunnel (NCT01632215), a randomized phase 3 study. The primary outcome is Pain Intensity.

2. Clinical Question

The study’s stated purpose was to evaluate the postoperative analgesic effect of preoperative gabapentine for carpal tunnel syndrome surgery. The primary endpoint was pain intensity, a continuous measure, rather than a count of participants crossing a threshold or a time-to-event outcome. The registry record also lists chronic pain as a secondary outcome, but does not provide a result for it in the posted outcome information.

The reported group means were 0.3 for preoperative gabapentine and 0.4 for the sugar pill at the stated 6-month assessment. The posted mean difference was 0.1, with a p-value of 0.826. These are descriptive and inferential summaries of the recorded endpoint; they do not, by themselves, establish whether any difference is important to patients. The registry record does not report a threshold for clinical importance or enough information to characterize how individual participants’ pain changed over time.

3. Trial Design

This was a phase 3, randomized, triple-masked, parallel-group trial listed as completed. The registry record gives an enrollment of 40 and identifies the sponsor as the Federal University of São Paulo. The listed study period began in 2010-03, with primary completion in 2011-08. The trial concerned carpal tunnel syndrome and the use of preoperative gabapentine in the context of surgery.

The two listed interventions were gabapentine and a sugar pill. The arm descriptions identify the gabapentine group as “Preoperative Gabapentine,” and the sugar-pill arm as the placebo group. The record describes the design as randomized and triple masked, but does not specify who was masked or how masking was maintained. It also does not report the randomization procedure, allocation concealment, treatment dose, exact timing, duration, adherence, or the details of the surgical care.

The primary outcome was pain intensity at 6 months. Its posted results include 18.0 participants for the preoperative gabapentine arm and 19.0 for the sugar-pill arm. The registry record does not explain how these outcome counts relate to the enrollment figure, describe participant flow, or give reasons for any difference between enrollment and the reported outcome counts. No analysis population definition is reported in the registry record.

Randomization is intended, in general, to make treatment groups comparable on average with respect to factors that could affect the outcome. Masking can reduce the influence of treatment expectations on care or outcome assessment. These are general design principles, not evidence about whether the particular procedures were successful in this trial. The registry record does not provide enough detail to assess the implementation of randomization or masking.

4. Participant Flow and Accounting

The registry reports how participants moved through each stage of the study. All counts below are exactly as posted; missing counts are not inferred from other stages.

Overall Study

5. Baseline Characteristics

The table reproduces the baseline characteristics posted in the registry, which describe the participants analysed at baseline and help show whether the randomized groups were comparable.

6. Primary Endpoint

The registry lists the primary outcome as Pain Intensity, measured over 6 months.

The primary endpoint was pain intensity, measured at 6 months as a numerical score from 0 to 10. The description states that zero means no pain and 10 is the more intense pain. The outcome is listed as a continuous measure summarized by its mean and standard deviation. The posted mean was 0.3 in the preoperative gabapentine group and 0.4 in the sugar-pill group.

A continuous pain score preserves variation across the scale rather than reducing responses to categories such as “pain” and “no pain.” Comparing group means therefore describes the average score in each arm, while the standard deviations describe how dispersed individual scores were around each group’s mean. This is a general explanation of the statistical summaries; the registry record does not report the distribution of individual scores.

Pain intensity is relevant to the stated study purpose because the trial sought to evaluate postoperative analgesia. A measurement at 6 months addresses the outcome at that recorded follow-up time; it does not describe pain at every point after surgery. The record does not give a baseline pain measure, a minimum clinically important difference, or a rule for interpreting particular score changes. As a result, the posted means and their comparison can be described, but the registry record does not establish what size of difference would be important to patients or clinicians.

7. Results Posted in the Registry

• Sugar Pill: mean 0.40 versus 0.30 on control; difference 0.10 (95% CI -0.79 to 0.99), standardized effect d = 0.07; z-test p = 0.826.

8. Statistical Interpretation

9. Statistical Methods Explained

The primary outcome is a continuous measure, so the posted comparison uses group means. In general, a mean is the arithmetic average of recorded scores. It gives a compact description of the center of a group’s measurements, but it does not show the full distribution, the range of scores, or how many participants had any particular score. Here, the registry record reports a mean of 0.3 for preoperative gabapentine and 0.4 for the sugar pill.

What does the standard deviation describe?

The standard deviation is a measure of spread around a group mean. The posted standard deviations are 0.8 for preoperative gabapentine and 1.8 for the sugar-pill group. In general, a larger standard deviation indicates that measurements are more dispersed around the mean. It is not the same as the standard error, and it is not a confidence interval for the mean. The registry record does not provide individual observations, so the shape of either group’s score distribution cannot be assessed from these summaries alone.

How should the mean difference be read?

The posted mean difference is 0.1. In general, a difference between means summarizes how far apart the group averages are, using the original outcome scale. The arm means indicate a higher average score in the sugar-pill group than in the preoperative gabapentine group; the posted difference is expressed for the comparison against the stated control arm. Because the scale runs from zero to 10, the difference is in units of that pain scale. The record does not report a threshold for deciding whether a particular difference is clinically meaningful.

What does the p-value address?

The unadjusted p-value is 0.826. In general, a p-value quantifies how unusual data at least as inconsistent with a null model would be if that model and the analysis assumptions were correct. It does not give the probability that the null model is true. It also does not measure the size of the difference or the importance of the outcome to participants. The word “unadjusted” indicates that the posted value is not adjusted for multiplicity; the registry record does not describe a broader multiplicity procedure.

Statistical tests and intervals depend on assumptions about the outcome and analysis. For a comparison of means, the model may rely on assumptions about independence and the distribution of errors, among other details. The registry record does not report the specific test, model, diagnostic checks, or transformations, so those details cannot be assessed. In general, marked skewness or influential observations can make simple mean-based inference less reliable, especially when the outcome counts are limited. The posted standard deviations alone cannot establish whether those concerns apply.

Why does the interval matter?

A confidence interval is a range of values produced by a method that, over repeated studies under its assumptions, has a specified coverage property. It is a way to express uncertainty around an estimated difference, rather than relying only on a single point estimate or a p-value. The posted interval extends from below zero to above zero. In general, an interval for a difference that crosses zero includes both no difference and values in either direction, according to that interval method. The precise confidence level and interval construction are not reported in the registry record, so they should not be inferred.

The reported comparison also includes a pooled standard deviation of 1.406. In general, a pooled standard deviation combines group variability for use in a standardized effect calculation under specified assumptions. The registry record reports this quantity, but it does not identify the exact formula or provide an adjusted analysis. The standardized effect estimate is not needed to interpret the mean difference in the original pain-score units.

The record lists one analysis and two outcome measures, with chronic pain as a secondary outcome. It does not report whether the primary analysis followed a prespecified statistical analysis plan, whether there were interim analyses, how missing observations were handled, or which participants were included in each analysis. Those items affect how fully a reader can evaluate the inferential result and are not recoverable from the group means and standard deviations alone.

In general, over-dispersion means that observed variability is greater than a statistical model expects. It can make model-based standard errors too small if the model fails to account for that extra variability. The posted outcome is a continuous score, and the registry record does not report that an over-dispersion model was used or that over-dispersion was identified. This concept is therefore general context, not a claim about this trial.

10. Confidence Intervals and Precision

The estimated mean difference is 0.1. The interval reported with the comparison extends across zero, from a negative value to a positive value. This means that the estimate is uncertain about direction: values favoring either group are compatible with the interval. The interval is also broad relative to the small point estimate, so the result does not pin down the size of the difference precisely.

A confidence interval is useful because it presents a range of values that remain compatible with the data under the interval method and its assumptions. It should not be read as a range containing a fixed proportion of individual participants’ pain scores. Nor does an interval crossing zero prove that there is no group difference. It indicates that the data do not distinguish zero from the range of differences included by the interval with the method used.

The registry record reports the group standard deviations as 0.8 and 1.8, and the reported outcome counts as 18.0 and 19.0. The greater spread in the sugar-pill arm is part of the observed data. In general, higher variability makes group means harder to estimate precisely, while larger outcome counts can improve precision; these factors work together, and the posted interval summarizes uncertainty for the comparison.

The confidence level and construction method for the reported interval are not reported in the registry record. No clinical-importance margin is reported either. Consequently, statistical uncertainty can be described, but the interval cannot be compared with a stated clinical threshold. The p-value of 0.826 is consistent with the interval’s inclusion of zero, but it should not replace inspection of the interval’s width and direction.

11. Planning a Similar Trial: Sample Size

This is a planning illustration, not a re-evaluation of the original trial. It asks: if a new study wanted to detect a difference like the one observed, or a smaller one, how large would it need to be? It is deliberately not “observed power” of the finished trial, which is not a meaningful quantity.

The planning figures are participants per arm, not total event counts. The event-count convention used for time-to-event outcomes does not apply to this continuous pain-score comparison. The displayed planning values are illustrative calculations based on the observed effect, not the trial’s original design assumptions. The trial’s own sample-size assumptions are not reported in the registry record.

The planning calculation assumes a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. It considers detecting the stated fraction of the observed effect. For the observed effect, the figures are 3,104 participants per arm for 80% power and 4,155 participants per arm for 90% power. For planning for 75% of the observed effect, the figures are 5,518 participants per arm for 80% power and 7,387 participants per arm for 90% power. For planning for 50% of the observed effect, they are 12,415 participants per arm for 80% power and 16,620 participants per arm for 90% power.

In general, power is the probability that a specified analysis will reject its null hypothesis when a particular effect is true, under the assumptions used in the calculation. The 80% and 90% columns therefore represent different target probabilities for detecting the assumed effect. A higher target power requires more participants when the other assumptions remain the same. Smaller effects are harder to distinguish from random variation, so the planning figures increase as the assumed effect gets smaller.

These values are planning illustrations, not observed power. They should not be read as a judgment about the trial’s conduct or as evidence that the trial was designed to detect the observed difference. The original sample-size justification and design assumptions are not reported in the registry record.

12. How the Sample Size Is Derived

Power is the probability that a study of a given size rejects the null hypothesis when a specific true effect exists. It depends on four things: the effect to detect, the variability of the outcome, the significance level (here a two-sided α of 0.05, so z_1−α/2 = 1.960) and the desired power (z_1−β = 0.842 for 80% and 1.282 for 90%).

Formula

n is the number of participants in each arm, Δ is the difference in means to detect and σ is the common standard deviation.

Worked calculation for this trial

1. Observed difference Δ = 0.100; pooled standard deviation σ = 1.406.
2. n = 2 × (1.96 + 0.842)² × 1.406² / 0.100² = 3,103.7, rounded up to 3,104 participants per arm for 80% power.

The smaller-effect rows in the table above repeat the same calculation with the effect multiplied by 0.75 and 0.5 (for ratios, applied to the logarithm of the ratio).

R code

alpha <- 0.05
z_a   <- qnorm(1 - alpha / 2)          # two-sided
delta <- 0.1                      # observed difference in means
sigma <- 1.40610911992735                     # pooled standard deviation
n_per_arm <- function(power, frac = 1) {
  ceiling(round(2 * (z_a + qnorm(power))^2 * sigma^2 / (delta * frac)^2, 6))
}
for (f in c(1, 0.75, 0.5))
  cat(sprintf("%3.0f%% of observed effect: n per arm = %s (80%% power), %s (90%% power)\n", 100 * f,
              format(n_per_arm(0.80, f), big.mark = ","), format(n_per_arm(0.90, f), big.mark = ",")))

# Check against R's built-in function (t-based, so very slightly larger)
cat("power.t.test:", round(power.t.test(delta = delta, sd = sigma, power = 0.80)$n, 1), "\n")

# Simulation: exact two-sample t-test using simulated means and variances
set.seed(1632215)
n    <- n_per_arm(0.80)
reps <- 20000
m_diff <- rnorm(reps, delta, sigma * sqrt(2 / n))             # difference in sample means
s2     <- sigma^2 * rchisq(reps, 2 * (n - 1)) / (2 * (n - 1))   # pooled sample variance
tstat  <- m_diff / sqrt(s2 * 2 / n)
cat(sprintf("Simulated power at n = %s per arm: %.3f (%d replicates)\n", format(n, big.mark = ","), mean(abs(tstat) > qt(0.975, 2 * (n - 1))), reps))

Output from running the code

100% of observed effect: n per arm = 3,104 (80% power), 4,155 (90% power)
 75% of observed effect: n per arm = 5,518 (80% power), 7,387 (90% power)
 50% of observed effect: n per arm = 12,415 (80% power), 16,620 (90% power)
power.t.test: 3104.6 
Simulated power at n = 3,104 per arm: 0.799 (20000 replicates)

The simulation draws many sets of group means and variances at the planned sample size and counts how often the two-sample t-test rejects the null. It should land close to the target power; at very small required sample sizes it can fall below it, because the normal approximation is rough there.

13. Multiplicity, Missing Data and Analysis Populations

The registry record lists pain intensity as the primary outcome and chronic pain as a secondary outcome. It reports two outcome measures and one analysis, but does not provide a result for the secondary outcome in the registry entries. The unadjusted p-value for the primary comparison is 0.826. No multiplicity adjustment or prespecified hierarchy among outcomes is reported in the registry record.

In general, when researchers test multiple outcomes or perform multiple comparisons, the chance of at least one small p-value arising by chance can increase. Methods for handling multiplicity can control that risk, but the appropriate approach depends on the trial’s planned questions and analysis. The registry record does not report whether any such approach was used, so the primary p-value should be understood as the posted unadjusted result.

The record gives outcome counts of 18.0 for preoperative gabapentine and 19.0 for the sugar pill, alongside an enrollment figure of 40. It does not provide a participant flow diagram, counts of missing measurements, reasons for missingness, or the method used to handle missing data. It is therefore not possible to determine from the registry record whether all enrolled participants contributed to the primary analysis or whether missing data could have affected the comparison.

The analysis population is also not described. The record does not say whether the analysis followed an intention-to-treat principle, included only participants who completed treatment, or used another population definition. In general, the analysis population and missing-data approach can influence interpretation, particularly if inclusion depends on post-randomization events or outcome availability. Whether any such issue arose in this trial is not reported in the registry record.

The registry record does not describe a statistical analysis plan, interim analyses, adjustment for baseline values, or any sensitivity analyses. These details matter for assessing whether the posted analysis followed a prespecified approach and how robust the result may be to different assumptions. Their absence from the registry record does not demonstrate that the procedures were or were not used.

14. Safety Results

The registry reports serious adverse events as affected participants divided by participants at risk.

Other (non-serious) adverse events

Participants with at least one non-serious event at or above the registry's 0% reporting threshold.

The serious adverse event information reports 20 participants at risk in each arm and zero affected by serious adverse events in each arm. The record also reports zero affected by other adverse events in each arm, using the listed at-risk counts of 20 for each group. These figures are registry counts for the categories reported; they do not provide descriptions of individual events because no serious-event terms are listed.

The deaths fields are null for both groups. Death counts are therefore not reported in the registry record; a null value should not be interpreted as a reported zero. The available figures also do not give event timing, severity grading beyond the serious-event category, relationship to treatment, duration, or the method used to collect adverse-event information.

In general, absence of recorded events in a registry category is not enough to establish that there is no safety risk. The ability to detect uncommon events depends on the size and duration of follow-up, and the registry record does not report a detailed safety follow-up period or collection protocol. These safety counts should be read narrowly as the reported adverse-event figures, not as a complete assessment of safety or a treatment recommendation.

15. What the Result Does, and Does Not, Mean

The posted primary comparison describes pain intensity at 6 months after carpal tunnel syndrome surgery. The group means are 0.3 for preoperative gabapentine and 0.4 for the sugar pill, with a mean difference of 0.1 and an unadjusted p-value of 0.826. The interval around the mean difference crosses zero. As a statistical summary, this result does not provide strong evidence of a difference between the two group means, and its interval leaves uncertainty about both direction and size.

That interpretation is narrower than saying the treatments are equivalent. A non-small p-value does not prove that the groups have the same outcome. Equivalence or noninferiority requires a design and analysis that use a prespecified margin; such a margin is not reported in the registry record. Likewise, whether a difference matters to patients cannot be determined without a clinical-importance threshold, which is not reported.

The result applies to the recorded pain-intensity measure at the stated 6-month time point and to the participants and procedures represented in the registry record. It does not establish what happened at other follow-up times, whether chronic pain differed, or how participants’ pain changed from before surgery. The registry record does not report those details in the primary outcome information.

The posted result also does not settle questions about treatment dose, timing, adherence, participant selection, or broader surgical care. These details are not reported in the registry record. The outcome counts and missing-data methods are not fully explained, which limits assessment of the analysis population and potential effects of missing outcomes.

Overall, the observed mean difference is small in scale units, but the interval indicates that the estimate is imprecise. The result should be interpreted as an uncertain comparison rather than evidence that gabapentine does or does not work. It is not medical advice and does not determine an individual treatment choice.

16. Why This Trial Matters Statistically

This trial illustrates why a point estimate, an uncertainty interval, and a p-value answer different questions. The mean difference of 0.1 describes the observed separation between group averages on the pain scale. The interval indicates substantial uncertainty around that estimate and crosses zero. The p-value of 0.826 describes the result under a statistical testing framework; it is not a measure of the probability that the treatment has no effect.

The example also shows why a result that is not statistically persuasive cannot automatically be treated as evidence of equivalence. A wide interval can include no difference while also including differences in either direction. Without a prespecified clinical threshold, readers cannot decide from the registry record whether the range of plausible values would be negligible or important.

Variability is relevant to precision. The reported standard deviation is 0.8 in the preoperative gabapentine arm and 1.8 in the sugar-pill arm. This difference in spread helps explain why group means need to be considered alongside uncertainty rather than read as exact quantities. The registry record does not provide individual scores or distributional diagnostics, so the summaries cannot show whether unusual observations or an asymmetric distribution influenced the result.

The planning illustrations demonstrate a further statistical principle: detectable effect size and sample size are linked. The calculations require substantially more participants per arm when planning for a smaller fraction of the observed effect. They are not a reconstruction of the trial’s original design, whose assumptions are not reported in the registry record, and they are not observed power.

Finally, transparent reporting of analysis populations, missing data, multiplicity, and safety follow-up is necessary for a fuller appraisal. Several of those details are not reported in the registry record. This trial is statistically instructive because its concise summaries can be interpreted accurately only when their uncertainty and reporting limits are kept in view.

17. Limitations

• The posted summary statistics are the only data these results rest on; patient-level data, full protocols and statistical analysis plans are not available.
• Recalculated results are simple, unadjusted calculations from posted summaries. They do not replace the sponsor's prespecified analysis.

The registry record does not report the gabapentine dose, the exact interval between administration and surgery, or treatment duration. It also does not provide detailed eligibility criteria, participant characteristics, or descriptions of the surgical procedures. These omissions limit assessment of how closely the trial population and treatment regimen can be characterized.

For the primary outcome, the record reports the 6-month mean and standard deviation by arm but does not describe baseline pain scores, individual score distributions, or the method used to handle repeated measurements. It does not state the statistical test or model, the confidence level for the reported interval, or whether assumptions were checked. The clinical importance threshold for the pain scale is also not reported.

Enrollment is listed as 40, while the primary outcome counts are 18.0 and 19.0. The registry record does not explain this relationship, give participant flow, or identify the reasons for unavailable measurements. Missing-data procedures and the primary analysis population are not reported, so the potential influence of incomplete outcome data cannot be assessed.

The record lists chronic pain as a secondary outcome but does not provide its result in the posted outcome information. It does not describe a multiplicity strategy, a statistical analysis plan, interim analyses, or sensitivity analyses. The masking label is triple, but the record does not identify which roles were masked or how masking was maintained.

Safety reporting is limited to the listed adverse-event counts. Deaths are not reported in the registry record, and event timing, follow-up procedures, and other details are not described. These reporting limitations constrain interpretation; they do not establish that unreported procedures or analyses were absent from the study.

18. Related Tutorials

19. Related Statistical Calculators

20. Sources

• ClinicalTrials.gov: Preoperative Gabapentine for Carpal Tunnel — NCT01632215.
• Registry record last updated 2019-03-05; status last verified 2019-02.
• Results first posted 2014-06-04.
• Data retrieved from ClinicalTrials.gov on 2026-11-09.
• Group sizes, means, event counts and sponsor-reported p-values are reproduced as posted. Effect estimates, confidence intervals and p-values not attributed to the sponsor are independently calculated from the posted summary statistics.

21. Record Summary

The registry record describes a completed phase 3, randomized, triple-masked, parallel-group study of preoperative gabapentine versus a sugar pill for carpal tunnel syndrome surgery. Enrollment is listed as 40. The primary outcome was pain intensity at 6 months, scored from 0 to 10.

The posted mean scores were 0.3 for preoperative gabapentine and 0.4 for the sugar-pill group. The mean difference was 0.1, with an unadjusted p-value of 0.826 and an interval that crosses zero. The outcome counts were 18.0 and 19.0, and the registry record does not explain participant flow or missing-data handling.

Chronic pain is listed as a secondary outcome, but its result is not reported in the registry record. Serious and other adverse-event counts list zero affected in each group; deaths are not reported in the registry record. The trial’s original sample-size assumptions, analysis population, statistical plan, and clinical-importance threshold are not reported.